Method and apparatus for baking



March 24, 1964 E. FREROTTI ETAL 3,

METHOD AND APPARATUS FOR BAKING SOEDERBERG ANODES FOR ELECTROLYSIS CELLS Filed June 26. 1961 FIG. I

Wig I IN VEN TOR. E dmondo Frerolt/ Gua/fiero Landucci United States Patent METHOD AND APPARATUS FOR BAKING SOEDERBERG ANUDES FOR ELECTROLY- SIS CELLS Edmondo Frerotti and Guaitiero Landucci, Eolzanc, Italy,

assignors to Montecatini, Societa Generale per llindustria Miner-aria e Chimica, Miian, Italy, a corporation of Italy Filed June 26, 1961, Ser. No. 119,464 Claims priority, application Italy June 28, 1960 3 Claims. (Cl. 204-67) The present invention relates to a method and apparatus for baking Soederberg anodes for electrolysis cells of the type used in the manufacture of aluminum, and more particularly to such method and apparatus wherein baking of the Soederberg anodes is carried out by means of alternating or direct current outside of the electrolytic cell itself.

Reference is made to the McGraw-Hill Encyclopedia of Science and Technology, 1960', vol. 1, pages 290-294, for a discussion of Soederberg electrodes and prior art methods of their manufacture. Reference is also made to copendi-ng application Serial No. 119,465 of Landucci, filed June 26, 1961, and Serial No. 119,466 of Frerotti et. al., filed June 26, 1961.

As is well known, Soederberg self-baking anodes are widely employed in the industry in order to lead the current to the electrolyte of electrolytic cells, and such anodes are substantially constituted by a sheet-metal jacket whose horizontal cross section corresponds to the anode section. In the customary method of manufacture, a paste-like material composed of coal and pitch in definite proportions is charged into the sheet-metal jacket, and electric current is passed through the paste-like material by means of metallic rods, called stubs, which penetrate into the paste-like material. This paste-like material is then baked by the heat of the cell, so that when the anode electrode is progressively lowered as it is consumed during the electrolysis, the lower part of the anode, on contact with the electrolytic bath, becomes hard-baked, which is the best condition for the anode leading the electric current to the interpolar zone and the best condition for the anode to take a part in the electrolytic reduction of alumina (A1 0 in the production of metallic aluminum.

When a new electrolytic cell is started up, the electrode must be previously baked. Such baking is customarily carried out by lowering the anode into the cavity formed by the cathodic lining of the electrolytic cell, until the anode rests on the carbonaceous bottom, upon a previously interposed bed of powdered coal. The stubs or metallic rods are kept at the desired height by means of previously baked coal shims or cylinders. According to this known method, the lower part of the electrode is contained within a sheet-iron tray or container, whose task it is to contain the coal and pitch mixture or paste in its fluid phase during the baking operation. In such cus tomary method of baking the anodes, the direct current utilized is conveyed to the electrode through the same circuit from which the electrolytic cell is subsequently fed for the electrolysis operation. Such direct-current baking, i.e., carried out in situ, exhibits a number of drawbacks, which may be summarized as follows:

Production lags occur, occasioned from the period of baking time required which cannot be accelerated beyond a certain limit without danger of impairing the cathodic bottom, thus tying up the electrode cell and its electrolysis circuit :when it might otherwise be used for electrolytic production. For the same reason, a greater labor burden is required. Furthermore, excessive smoke is released in the room containing the cells during such baking in 3,125,326 Patented Mar. 24, 1964 2 situ. Such prior art method also incurs localized electric and thermal stresses in the cathodic bottom with a resulting shorter life of the bottom and a concentration of electrical current.

The above-mentioned drawbacks are eliminated when the baking operation is carried out according to the present invention. According to the invention, the Soederberg anode, instead of being prepared and baked on the cell itself, is prepared and baked in separate apparatus or baking rooms which are removed from the cell. Either alternating or direct current may be used, and the baking is carried out in a separate resistor furnace, the anode being formed from fresh electrodic paste. The apparatus comprises a container base of insulating masonry, a metallic tray for containing the fresh electrodic paste, and cross beams and contact stubs are provided for the electrical and mechanical connection of the electrode.

In the accompanying drawings:

FIG. 1 represents an elevation view partially in section of a baking apparatus according to the invention;

FIG. 2 is a plan view of the apparatus according to FIG. 1;

FIG. 3 is a longitudinal section of the tray of FIG. 1; and

FIG. 4 is a transverse cross section through the detail of FIG. 3.

The baking room and apparatus according to the invention, as shown in the accompanying drawings, comprises essentially a container base a made up of insulating masonry, an electric transformer b of variable ratio, a cross beam 0 supported by trestles or frame structure d, bus bars e for conveying the electric current from the transformer b to the baking apparatus, and clamps f for fastening the stubs or metallic rods g to the cross beam 0 and bus bars e.

The following procedure is preferably adopted for carrying out the baking operation. A sheet-metal tray h (FIG. 3) is provided, having a horizontal cross section corresponding to the section of the electrode to be produced, and the tray h is arranged within the empty space in the base of the container a. Inert insulating powder 1', for example alumina (A1 0 is interposed between the tray h and the insulating masonry of the container base a.

The longitudinal portion of tray h is insulated toward the inside by means of sheets of insulating material m, such as asbestos board, as shown in the longitudinal section of FIG. 3 and the transverse cross section of FIG. 4.

The stubs g, fastened to clamping lugs f, are arranged at the desired height by interposing small cylinders l of coal or other carbonaceous material between the lower end of the stubs and the tray h. The clamping lugs f are set up in the precise position corresponding to that of the anodic terminal clamps of the electrolysis cell. The tray h is then filled with fresh electrodic paste. Current, either direct or alternating, is conveyed to the paste mix ture, starting with the lowest current values and increasing the current gradually as the baking operation proceeds. The current is transmitted to the stubs of the longitudinal portion of the electrode. Consequently, the current enters from these stubs, passes into the carbon-like mass of the electrode paste, and leaves from the stubs of the other longitudinal side of the electrode, as shown in FIG. 2. The arrows show the direction of the current travel. The insulating sheets m aim at preventing a current short circuit through the tray h.

In order to further increase the capacity of the tray h, a small jacket 21 may be provided (FIGS. 1, 3 and 4) which can be opened into two halves, as shown in FIG. 4. The jacket n is clamped around the tray sides in order to obtain a heightening structure, which is removable from the sides of the tray 11.

When the baking operation is completed, the jacket 11 may be readily taken apart and recovered for further 4 electrical conducting members, placing within said tray in electrical contact with said members and in electrical contact with a portion of said tray a Soederberg-type carbonaceous paste, passing an electric current through use in baking of further ode 5 said conducting members and through said tray and In place of the jacket n, the definitive anodic jacket to through said paste so that the paste itself forms a currentbe eventually used in the electrolytic cell itself may be carrying path to produce a resistance-heating effect, gradprovided and used in forming the anode. A plurality of ually increasing said electric current from a relatively aking apparatuses may also be used, arranged in series low value to a higher value until said paste acquires a with one another and fed from the same transformer. hard baked layer, and transferring the paste mass thus Advantages of the described process and apparatus may baked to an electrolytic cell and connecting said mass as be summarized as follows. the anode thereof.

The invention provides speedy baking of the electrodes, 2. A process for starting an electrolytic furnace for since it is not restricted by the possibilities of impairing the production of aluminum, comprising the steps of the cathodic bottom, as in the prior art methods. A replacing an electrically insulating refractory powder within duced labor burden is incurred, and smoke from the a masonry container, positioning over said powder ametal baking operation is eliminated from the electrolytic cell tray having a cross section corresponding to the section workrooms. Mechanical and thermal stresses, caused by of the electrode to be produced, positioning on the bottom the prior art methods of baking of the cathodic bottom, and lower side-wall portions of said tray a layer of elecare prevented. Opportunity is also provided for using trical insulating material, positioning within said tray and alternating current, more readily carried and more easily against said layer of insulating material a plurality of transformable according to the variable loads in accordelectrical conducting members, filling said tray with a ance with the specific requirements of the baking opera- Soederberg-type carbonaceous paste until said paste is in tion. A lower cost of baking is thus incurred, because electrical contact with said members and with the upper of the lower cost of alternating current as compared with side-wall portions of said tray, passing an electric current direct current. through said conducting members and through said paste In Table I are shown some typical characteristics of so that the paste itself forms a current-carrying path to the operation carried out according to the invention by produce a resistance-heating eifect, gradually increasing Way of example. said electric current from a relatively low value to a higher Table 1 Volt Current Temper. Baking (Amperes) Conof elee- Elec- Baking Anodic durasumed trodo trode height element tion, energy, bottom height, mean of hours Mini- Maxi- Ini- Final, kwh. mean cm. value,

mum, mum, tin], a. Value, cm.

v. v. a. C.

27 ka 72 a 10 3,000 10,000 3,200 700 so 30 04 ka 132 10 15 0, 000 10,000 10,000 700 60 30 As shown in Table I, the two listed examples relate to value until said paste acquires a hard baked layer, transtests with anodes of 27 ka. and 64 ka., respectively. The ferring the paste so baked to a Soederberg cell and con- .values relate to baking of the anodes in alternating curmeeting same as an electrode to the cell for the production rent using an anode element equal to about half of a of aluminum. normal anode employed in the electrolytic furnace cell. 3. Apparatus for pro-baking of Soederberg-type elec- In other words, the test was carried out on half-size trodes, comprising a resistance furnace having a masonry anodes. Baking operations were performed With three container base of electrically insulating material, a elements at a time for anodes of 27 ka. and with two metallic tray for containing fresh Soederberg carbonaceous elements for anodes of 64 ka. employing a monphase electrodic paste, and having a cross section corresponding 600 kva. transformer. When a transformer of higher to the electrode to be produced, insulating material formpower is used, the baking of the furnaces may be eX ing an electrical insulating layer against only the lower pedited by several days. side and bottom portion of the inner surface of said tray,

The values above listed in Table I apply in an analogous contact stubs positioned within said tray with their bottoms manner for direct-current operation as well as for alteragainst said layer and mutually spaced for transmitting nating current. electrical current along a generally horizontal path through It will be obvious to those skilled in the art, up the electrodic paste so that the paste itself forms the studying this disclosure, that the methods and apparatus current-carrying path so as to produce a resistance-heating according to our invention can be modified in various effect, supporting structure for the contact stubs, and respects and hfince y be embodied in PIQCfiSias and means for mechanically connecting and supporting the devices other than particularly illustrated and described t b t id u ortin tru ture. herein, without departing from the essential features of our n invention and within the scope of the claims annexed References Cited m the of this Patent harem UNITED STATES PATENTS We Claimi 2,378,142 Hurter June 12, 1945 1. A method for baking carbonaceous electrodes f 2,75 ,9 4 Liles Aug 14, 195 Soederberg electrolytic cells outside of the elec rolytic 7 9 Tommdstad 3, 1959 cell, which comprises the steps of arranging on an l 2,937,9 0 s t et 1 May 24, 19 0 trically non-conducting refractory base a metal tray a 2,938,343 D V d May 31, 1960 ing a horizontal cross section corresponding to the sec i n ,959,52 Graybeal v, 3, 19 0 of the electrode to be produced, positioning on thelinn r ,755 schmitt July 10, 9 2 bottom surface of said tray a layer of electrical insu ating material, positioning within said tray and against the FOREIGN PATENTS upper .side of said layer insulating material a plurality of 1 ,150 Sw d n D 399 

1. A METHOD FOR BAKING CARBONACEOUS ELECTRODES FOR SOEDERBERG ELECTROLYTIC CELLS OUTSIDE OF THE ELECTROLYTIC CELL, WHICH COMPRISES THE STEPS OF ARRANGING ON AN ELECTRICALLY NON-CONDUCTING REFRACTORY BASE A METAL TRAY HAVING A HORIZONTAL CROSS SECTION CORRESPONDING TO THE SECTION OF THE ELECTRODE TO BE PRODUCED, POSITIONING ON THE INNER BOTTOM SURFACE OF SAID TRAY A LAYER OF ELECTRICAL INSULATING MATERIAL, POSITIONING WITHIN SAID TRAY AND AGAINST THE UPPER SIDE OF SAID LAYER INSULATING MATERIAL A PLURALITY OF ELECTRICAL CONDUCTING MEMBERS, PLACING WITHIN SAID TRAY IN ELECTRICAL CONTACT WITH SAID MEMBERS AND IN ELECTRICAL CONTACT WITH A PORTION OF SAID TRAY A SOEDERBERG-TYPE CARBONACEOUS PASTE, PASSING AN ELECTRIC CURRENT THROUGH SAID CONDUCTING MEMBERS AND THROUGH SAID TRAY AND THROUGH SAID PASTE SO THAT THE PASTE ITSELF FORMS A CURRENTCARRYING PATH TO PRODUCE A RESISTANCE-HEATING EFFECT, GRADUALLY INCREASING SAID ELECTRIC CURRENT FROM A RELATIVELY LOW VALUE TO A HIGHER VALUE UNTIL SAID PASTE ACQUIRES A HARD BAKED LAYER, AND TRANSFERRING THE PASTE MASS THUS BAKED TO AN ELECTROLYTIC CELL AND CONNECTING SAID MASS AS THE ANODE THEREOF. 